1. Field of the Invention
This invention relates to electronic engine control and, more particularly, to a method and system for controlling cam timing.
2. Background Art
Variable cam timing systems operate to vary the timing between the camshaft and the crankshaft to optimize engine performance over the entire range of engine operation. Systems such as that described in U.S. Pat. No. 5,117,784 to Schechter et al., assigned to the assignee of the present invention, vary the timing between the camshaft and crankshaft to achieve improved idle stability, expanded torque curve over the RPM (revolutions per minute) range of the engine, better control of emission gases, and possible elimination of external gas recirculation components and circuitry.
It is known that optimal cam timing for fuel economy and emissions may be achieved by determining the timing as a function of engine speed and air charge entering the engine in lbs/cylinder filling. Optimal cam timing for power may be achieved by determining the cam timing as a function of engine speed and throttle position. Either of the aforesaid control methods can generate cam timing to achieve satisfactory fuel economy, emissions and performance for a particular altitude, usually sea level. However, as the altitude at which a vehicle is operated increases, a control method calibrated for sea level operation provides less than optimal results because the air charge entering the engine at a given throttle position decreases.
In U.S. Pat. No. 5,609,126 to Cullen et al, assigned to the assignee of the present invention, a variable cam timing system which provides optimal fuel economy, emissions and performance at a variety of altitudes is disclosed. In that patent, a first intermediate camshaft phase angle is retrieved as a function of engine speed and air charge and a second intermediate camshaft phase angle is retrieved as a function of engine speed and throttle position. The first intermediate camshaft phase angle is compared to the second intermediate camshaft phase angle, and the cam phase angle is determined as a function of the camshaft phase angle which corresponds to the least amount of camshaft timing retard. Alternatively, an interpolator value based on engine speed and throttle position is used to interpolate between the first and second intermediate phase angles to obtain the desired cam phase angle.
Where the engine is equipped with independent intake and exhaust valve camshafts, it is desirable that a system be provided for independently adjusting the rotational position of one camshaft relative to the other and each relative to the engine crankshaft. Such dual independent cam timing offers fuel economy advantages but is inherently complex to optimize. For example, maximum spark for best torque (MBT) must be calculated and varied with valve timing. Accordingly, there is a need for a dual independent cam timing method that optimizes control system behavior while minimizing complexity.
It is an object of the present invention to provide an improved cam timing method for an engine having dual independent camshafts. In accordance with a preferred embodiment of the present invention, calibration values for intake valve closing (IVC) and valve overlap (OL) that provide best fuel economy and emissions over the operating range of engine speed and indicated torque are contained in respective ROM based schedule tables that form a set of stability limited (SL) tables optimized to provide best fuel economy. A second set of ROM based schedule tables is provided that contains calibration values for IVC and OL for optimal power (OP) over the operating range of engine speed.
A performance or power index (PI) value, that indicates the driver""s relative desire for fuel economy or power, is obtained from a ROM based lookup table. The PI value is a function of the percent of available indicated torque demanded or desired by the driver. The PI value is used to interpolate over the range of values in the SL tables to the values in the OP tables to calculate values of IVC and OL that are optimal for all altitudes and levels of driver demand.
By using the percent of peak available indicated torque as the input to the interpolator table, aggressive cam timing schedules with high residual fraction can be used for optimal fuel economy with good driveability. Higher residual fraction improves fuel efficiency by reducing the torque required to pump the air into the cylinder and by reducing heat transfer losses from the combusted mixture to the surrounding walls and engine coolant. Another benefit of high residual fraction is a reduction in NOx emissions. Also, driver demanded torque is a function of pedal position. As soon as the controller reads the driver demand the desired cam timing settings can be scheduled. It takes a finite time for the camshaft to position itself so this feed forward capability permits the camshaft to move as soon as possible.